The invention relates to digital time division multiple access (TDMA) radio systems, local intracell capacity allocation in such a radio system, and an intracell handover in such a radio system.
In digital Time Division Multiple Access (TDMA) radio systems, a plurality of mobile radio stations may utilize the same radio channel on a time division basis for communication with a base station. Communication on the radio channel takes place in a number of successive time slots, e.g. 8 time slots, allocated to the users as required. A mobile radio station is synchronized with a signal from the base station, and it transmits in accordance with this synchronization signal ("Sync") so that the signal from the mobile station is received at the base station in a time slot allocated for this particular mobile station. Mobile stations, however, may be located at different distances from the base station, whereby the propagation delay caused by this distance has to be taken into account in the synchronization of the timing of transmission of each mobile station with the base station so that the signal transmitted by each mobile station will be received at the base station in the right time slot. For this purpose, the base station measures the time difference between its own transmission and the transmission received from each respective mobile station, on the basis of which it defines a suitable timing advance for the respective mobile station. Each mobile station uses this timing advance to advance the time of its transmission relative to a basic time instant defined by the sync received from the base station. Various intrasystem features limit the timing advance to a certain maximum. This maximum of the timing advance, in turn, dictates the maximum cell size that any respective base station of the system is able to serve. In the pan-European mobile radio system GSM (Global System for Mobile Communication), for instance, the timing advance may vary in the range between 0 and 233 .mu.s, which corresponds to a cell size with a maximum radius of 35 km.
In general, a cell in a radio system offers the same level of service within the entire area of the cell. In some cases, however, it may be necessary to allocate part of the radio capacity of the cell, either permanently or temporarily, merely to a certain area within the cell. It may be necessary to concentrate the capacity temporarily, for instance in cases of emergency or catastrophy, or at peak hours of an important traffic area (e.g. an airport). There have previously been attempts to allocate intracell radio capacity to some extent by the use of a sectored cell and directional antennas, but these means do not allow the capacity to be allocated to a certain geographical object in a sufficiently flexible, efficient and accurate manner.